Acute ischemic syndromes involving arterial blood vessels, such as myocardial infarction, or heart attack, and stroke, frequently occur when atherosclerotic plaque ruptures, triggering the formation of blood clots, or thrombosis. Plaque that is inflamed is particularly unstable and vulnerable to disruption, with potentially devastating consequences. Therefore, there is a strong need to detect and locate this type of plaque so that treatment can be initiated before the plaque undergoes disruption and induces subsequent life-threatening clotting.
Researchers, acting on the theory that inflammation is a factor in the development of atherosclerosis, have discovered that local variations of temperature along arterial walls can indicate the presence of inflamed plaque. The temperature at the site of inflammation, i.e., the unstable plaque, is elevated relative to adjacent plaque-free arterial walls.
Using a tiny thermal sensor at the end of a catheter, the temperature at multiple locations along an arterial wall were measured in people with and without atherosclerotic arteries. In people free of heart disease, the temperature was substantially homogeneous wherever measured: an average of 0.65° F. (0.36° C.) above the oral temperature. In people with stable angina, the temperature of their plaques averaged 0.19° F. (0.11° C.) above the temperature of their unaffected artery walls. The average temperature increase in people with unstable angina was 1.23° F. (0.68° C.). The increase was 2.65° F. (1.47° C.) in people who had just suffered a heart attack. Furthermore, temperature variation at different points at the plaque site itself was found to be greatest in people who had just had a heart attack. There was progressively less variation in people with unstable angina and stable angina.
The temperature heterogeneity discussed above can be exploited to detect and locate inflamed, unstable plaque through the use of cavity wall profiling apparatus. Once located, treatment can then be initiated upon the inflamed, unstable plaque region.
One such method of treatment involves cryo-therapy in which regions of tissue may be cooled to temperatures as low as −112° F. (−80° C.) by catheters inserted endovascularly into a patient. This form of treatment delivers energy to ablate the tissue in a manner which is generally safer and more effective than other conventional methods of treatment.
Various conventional endovascular catheters used for freezing, heating, or ablating tissue are well known. Certain catheters used for cooling and/or freezing typically employ one of several methods. One such method takes advantage of cooling by use of a phase change refrigerant which may enter a patient's body at ambient temperature and attains cooling by expansion within a cooling chamber disposed within the catheter body located at the selected treatment site. The wall of the cooling chamber is typically placed in contact with adjacent tissue to effect conduction cooling or ablation.
Another method involves utilizing the expansion of a phase change or high pressure coolant exiting from a nozzle within the catheter tip to create a highly turbulent flow region.
However, problems exist with conventional cooling and cryo-treatment devices. One such problem is that conventional devices usually exert an undue amount of force on the region of interest. If the region of interest cannot withstand these forces, it may be damaged. The inside walls of a healthy human artery are vulnerable to such damage. Furthermore, if inflamed, unstable plaque is present it may be ruptured by such forces.
Another problem with conventional devices is that they can only treat tissue at one specific location. In order to treat an entire tissue region of interest, one would need to move the cooling apparatus from location to location. If the cooling apparatus were placed on an angioplasty balloon, for instance, the balloon would need to be deflated, placed at the desired location, inflated and placed in contact with the tissue, and cooled, and so on for each treatment location. This can be very tedious, can increase the risk of damaging the vessel wall or rupturing vulnerable plaque, and may not treat the entire region of tissue.
Accordingly, there exists a need for a device which is able to cryogenically treat a predetermined region of tissue, such as a region of unstable plaque, efficiently and effectively without damaging the surrounding and underlying tissue.